High temperature cobalt and nickel-based superalloys are widely used to form certain components of gas turbine engines, including combustors and turbine vanes and blades. While high-temperature superalloy components are often formed by casting, circumstances exist where superalloy components are preferably or are required to be fabricated by welding. For example, components having complex configurations, such as turbine midframes and shroud support rings, can be more readily fabricated by welding separate castings together. Welding is also widely used as a method for restoring blade tips, and for repairing cracks and other surface discontinuities in superalloy components caused by thermal cycling or foreign object impact. Because the cost of components formed from high-temperature cobalt and nickel-based superalloys is relatively high, restoring/repairing these components is typically more desirable than replacing them when they become worn or damaged.
Superalloy components of gas turbine engines must generally be thermally stress-relieved before welding to relax residual stresses present from engine service, and then stress-relieved after welding to relax residual stresses induced during cool down from the welding operation. Heat treatment also provides stress relief by dissolution of a portion of hardening gamma prime (.gamma.') in .gamma.'-strengthened nickel-base superalloys. Generally, the heat treatment parameters will vary depending on the alloy of interest, the amount of residual stress relief and dissolution required, furnace design, component geometry and many other factors. The ramping rates, soak temperatures, hold times and cooling rates for stress relief and dissolution heat treatments are critical in order to obtain the desired stress relief without adversely affecting the superalloy and its properties.
In the past, pre-weld and post-weld heat treatments have been performed in large batch heat treatment furnaces to ramp and hold a group of components at a suitable heat treatment temperature. Following batch heat treatment, individual components are welded while being maintained at an elevated temperature (e.g., in excess of about 1500.degree. F. (about 815.degree. C.)) to improve welding yields. Welding is often performed in an enclosure containing a controlled atmosphere (e.g., an inert gas) using such welding techniques as tungsten inert gas (TIG) and laser welding processes. Heating is typically performed by induction or with the use of lamps, such as quartz halogen lamps.
While having certain benefits, drawbacks to the use of batch heat treatment processes include long heat treatment times due in part to the mass of the large batch furnace and the mass of the typically large number of components being heat treated. Additionally, long queuing times occur while batches are assembled as individual components are repaired. Therefore, use of batch furnace pre-weld and post-weld stress relief heat treatments represent a time delay to the flow of components through a welding line, and is an inefficient method to metallurgically condition components for welding.
In view of the above, it would be desirable if improved processing efficiency could be achieved for superalloy articles manufactured, restored or repaired by welding.